The lower the operating threshold of a varistor, the more effective the protection it provides against surge. To obtain a low varistor operating threshold, only a small number of varistor components should be associated in series. Unfortunately the energy absorbed by each varistor component is then high.
The limiting surge value against which protection can be provided by varistors is currently 1.6 p.u. (where 1 p.u.=(.sqroot.2/.sqroot.3) Un in peak value, where Un is the nominal phase voltage).
To limit surge below 1.6 p.u., while conserving acceptable energy dissipation in the varistor, proposals have been made, in particular in French patent application No. 90 02416, to insert the varistor by means of a delay system and a flexible drive member.
In French patent application No. 89 14433, proposals have been made to use a system having a compression spring and a traction spring.
These systems provide satisfaction, but they are relatively complex and expensive.
Non-published French patent application No. 79 08039 describes a compressed air circuit breaker including a system for inserting a resistor during interruption, in which the insertion electrode lies on the path of the arc. Attempts have been made to adopt this technique to inserting varistors, however the fact that the insertion electrodes are placed on the path of the arc means that they wear quickly, thereby increasing maintenance costs.
An object of the present invention is to provide a circuit breaker fitted with a device for inserting a varistor during a circuit breaker opening operation, which device is simple, reliable, cheap, and in particular does not include a delay system, nor any complex moving parts, nor an insertion electrode placed on the part of the arc.